It is well known that particles (e.g., atoms, molecules, cells) may have non-uniform concentrations of positive and negative charge. For instance, a molecule is made of multiple atoms that each include a positively charged center region known as a nucleus and a varying number of negatively charged subatomic particles known as electrons that exist in an outer negatively charged region around the nucleus. When the atoms combine to form the molecule, the forces associated with these two charged regions bind the atoms together. The shape of the molecule may result in a nonuniform distribution of charge, thereby producing respective concentrations of positive charge and negative charge. When this occurs, the molecule is called a polar or polarized molecule. Polarity can be induced in an otherwise non-polar molecule. Specifically, when a normally non-polar molecule is placed in a strong electromagnetic field, the negatively charged electrons are sometimes separated from the positively charged nuclei so that the otherwise non-polar molecule becomes polarized. For the purposes of the present disclosure, the terms “polar” or “polarized” are used to describe any particles that are naturally polar as well as any particles that are made polar by applying an electromagnetic field.
Since one region of a polar molecule is more positively charged and another region is more negatively charged, a force is exerted on such a molecule when it is placed into a non-uniform electromagnetic field (an electromagnetic field in which the field strength varies from one location to another). This force, known as a dielectrophoretic force, can cause the molecule to move. The exact direction of movement and velocity at which the molecule moves depends upon the particular characteristics of the molecule, the medium in which the molecule is located, and the electromagnetic field.
Specialized microscopes referred to as Scanning Probe Microscopes (SPMs) can be used to probe molecules on a scale of nanometers. One type of SPM is the Atomic Force Microscope (AFM). An AFM comprises a cantilever having a tip on one end that is scanned across a sample. Very small movements of the tip, based on variations in the profile of the surface of the sample over which the tip is scanned, are typically measured by a laser detection system and are used to generate an image of the surface profile of the scanned sample.
In some instances, the tip of an SPM need not actually make contact with the sample surface to be imaged. For example, in U.S. Pat. No. 5,936,237 to van der Weide, an SPM tip is configured as a conductor with a conducting shield around it. This configuration allows the SPM to perform a non-contact scan of a sample surface to provide an image of the surface profile based on electromagnetic field interaction between the tip and the surface. During such a scan, because no insulator separates the conducting tip from the sample, a distance is maintained between the sample and the conducting tip.
Another device commonly used with small particles is a micromanipulator. Micromanipulators are devices that move tools which are used to probe and manipulate samples on a cellular scale. Micromanipulators are used, for example, for microsurgery and other biological purposes. A micropipette is one example of a tool that can be used to probe and manipulate samples in connection with a micromanipulator. An optical microscope capable of viewing individual cells can be used to directly view a sample being manipulated by a tool coupled to a micromanipulator (hereinafter referred to as a “micromanipulator tool”).